More than 3.5 million people have been forced to flee Ukraine since Russian forces invaded on Feb. 24, according to the latest figures from the United Nations refugee agency.

The tally from the U.N. High Commissioner for Refugees (UNHCR) amounts to just over 8% of Ukraine's population -- which the World Bank counted at 44 million at the end of 2020 -- on the move across borders in 27 days.

More than half of the refugees are in neighboring Poland, UNHCR figures show.

-ABC News' Zoe Magee

At least 925 civilians, including 75 children, have been killed in Ukraine since Russian forces invaded on Feb. 24, according to the latest figures from the Office of the United Nations High Commissioner for Human Rights (OHCHR).

Meanwhile, at least 1,496 civilians, including 99 children, have been injured, OHCHR figures show.

The tallies are civilian casualties that occurred in Ukraine from Feb. 24 to March 20 and have been verified by OHCHR, though the agency cautioned that the true numbers are believed to be "considerably higher."

"Most of the civilian casualties recorded were caused by the use of explosive weapons with a wide impact area, including shelling from heavy artillery and multiple-launch rocket systems, and missile and air strikes," OHCHR said in a statement late Monday. "OHCHR believes that the actual figures are considerably higher, especially in Government-controlled territory and especially in recent days, as the receipt of information from some locations where intense hostilities have been going on has been delayed and many reports are still pending corroboration."

Russia claimed Tuesday that its troops have captured nine more localities in Ukraine.

According to a statement from the Russian Ministry of Defense, units of the Russian Armed Forces have advanced another 6 kilometers (about 3.7 miles) and have taken control of the southeastern village of Urozhaine in the Donetsk oblast, some 65 miles north of the besieged port of Mariupol where many civilians remain trapped under Russia bombardment.

Meanwhile, the defense ministry said Russia-backed separatist forces of southeastern Ukraine's disputed Donbas region have also advanced and captured eight more areas in the Donetsk and Luhansk oblasts.

Russian Deputy Foreign Minister Sergey Ryabkov on Tuesday denied allegations that Russia might be planning to use biological or chemical weapons in Ukraine.

"We have neither of these," Ryabkov told reporters in Moscow. "What the Americans are saying are malicious insinuations -- we've heard them all the time and we've given exhaustive answers to them for a long time. The problem is, the U.S. has no habit of listening to anyone but itself."

Ryabkov's comments came after U.S. President Joe Biden accused Russian President Vladimir Putin of falsely claiming that the United States and Ukraine are developing biological or chemical weapons for use against Russia -- rhetoric that Biden said shows Putin is considering using those types of deadly weapons in Ukraine.

"He's already used chemical weapons in the past, and we should be careful of what's about to come," Biden said Monday during remarks at the Business Roundtable's CEO Quarterly Meeting in Washington, D.C.

All Russian troops have left the Ukrainian cities of Kyiv and Chernihiv, withdrawing north toward the borders of Belarus and Russia to consolidate before likely redeploying to the Donbas region in eastern Ukraine, a senior U.S. defense official told reporters Wednesday.

But even with the Russians gone, the territory remains treacherous.

"There are some indications that they left behind mines and things like that, so the Ukrainians are being somewhat careful in some areas north of Kyiv as they begin to clear the ground and clear the territory and re-occupy it," the official said.

While the U.S. hasn't yet seen these troops redeploy elsewhere in Ukraine, it'll likely happen soon, according to the official. Ukrainian forces are preparing for a major fight in Donbas, the official said.

The official also said the Pentagon is "monitoring" an apparent nitric acid explosion in Ukraine's Luhansk region, which Russia blamed on Ukraine.

"We've seen the Russians claim that this was a Ukrainian attack on this. We do not believe that is true," the official said. "We do believe that the Russians are responsible, but exactly what they used when they did it, why they did it, what the damage is, we just don't have that level of detail," the official said.

The official also noted that a small number of Ukrainians currently in the U.S. for "professional military education" were pulled aside for a couple days of training on Switchblade drones, which the U.S. is sending overseas as part of its military aid, according to the official.

"Although it's not a very difficult system to operate, we took advantage of having them in the country to give them some rudimentary training on that," the official said.

-ABC News' Matt Seyler